There are many infectious diseases for which an effective vaccine has not yet been developed, and many of the currently available vaccines provide only partial protection against disease. Further, there are gaps in the vaccine field. Live vaccines produce stronger, broader, and more durable immunity than other types of vaccines. There is a need for a safer live vaccine vehicle, which will be unable to cause disease even in immunosuppressed individuals. There is also a need for vaccines that induce cell-mediated immunity and not just antibody-based immunity. And, there is a need to induce protective immune responses directly at the mucosal surfaces of the body, where most pathogens gain entry. Thus, there is a need for improved vaccines.